More than a dozen Democratic senators, including the top Senate Democrat, on Wednesday called on Sen. Al Franken (D-Minn.) to resign in the wake of multiple sexual misconduct allegations against him.

Sen. Kirsten Gillibrand was the first of Franken's fellow Senate Democrats to take that step and was quickly followed by more than two dozen others. The first batch of resignation calls came from female senators, followed by a slew of male Democrats and eventually the majority of the 48-member caucus.

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Franken plans to make an unspecified announcement on Thursday, according to his office. The state director for his Minnesota colleague, Democratic Sen. Amy Klobuchar, said in a statement that Klobuchar spoke with Franken directly early Wednesday and offered no further comment beyond pointing to Franken's planned announcement.

But the resignation of Franken, who was once regarded as a rising star in the Democratic Party and talked about as a potential presidential contender, seemed inevitable after top party officials and most of his Democratic colleagues called on him to step aside. Minnesota Public Radio reported that Franken will announce his resignation on Thursday, and at least two senators — Ron Wyden of Oregon and Chris Murphy of Connecticut — said they expect Franken to do so.

“Senator Franken should resign," said Senate Minority Leader Chuck Schumer of New York, who became close with Franken when he ran the Democratic Senatorial Campaign Committee during the 2008 cycle, when Franken first ran for office. “I consider Senator Franken a dear friend and greatly respect his accomplishments, but he has a higher obligation to his constituents and the Senate, and he should step down immediately.”

Franken’s Twitter account, however, said he has not made a final decision and that he was still speaking with his family.

After a cascade of female senators led by Gillibrand called on the second-term senator to step down, Sen. Bob Casey (D-Pa.) became the first male Democratic senator to do so. Others followed in rapid-fire sequence.

"Senator Franken's behavior was wrong," Minority Whip Dick Durbin (D-Ill.) tweeted. "He has admitted to what he did. He should resign from the Senate."

Some male Democratic senators, however, stopped short of urging Franken to step down.

“I believe in the Sixth Amendment to the Constitution that people are innocent until proven otherwise," said Sen. Bob Menendez (D-N.J.), whose federal corruption case ended in a mistrial last month. He said in a later statement that Franken has "admitted there’s truth to some of the allegations, and it seems that he is coming to terms with the implications for his ability to remain in the Senate."

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Menendez added: "We are at a moment of reckoning where women are finally feeling that they can speak out about sexual harassment in what should be professional workplace environments without shame or fear of reprisal."

Sen. Tim Kaine (D-Va.) said Wednesday that he spoke privately with the Minnesota senator but would hold off on making a formal statement on the allegations until Franken's announcement on Thursday. Other Democratic senators who did not issue calls for Franken to step aside include Sens. Joe Manchin of West Virginia, Jon Tester of Montana, and Catherine Cortez Masto of Nevada.

"While Senator Franken is entitled to have the Ethics Committee conclude its review, I believe it would be better for our country if he sent a clear message that any kind of mistreatment of women in our society isn’t acceptable by stepping aside to let someone else serve," Gillibrand said in a Facebook post earlier.

The calls came after POLITICO reported that a former Democratic congressional aide said he tried to forcibly kiss her after a taping of his radio show in 2006, three years before he became a U.S. senator. Franken, who had previously been accused by six other women of groping or trying to forcibly kiss them, denied the accusation.

Hirono said she "struggled with this decision because he’s been a good Senator and I consider him a friend. But that cannot excuse his behavior and his mistreatment of women."

McCaskill tweeted simply: "Al Franken should resign."

Murray said in a statement that she was "shocked and appalled by Sen. Franken’s behavior. It’s clear to me that this has been a deeply harmful, persistent problem and a clear pattern over a long period of time. It’s time for him to step aside."

And Harris tweeted, "Sexual harassment and misconduct should not be allowed by anyone and should not occur anywhere. I believe the best thing for Senator Franken to do is step down."

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Senate Majority Leader Mitch McConnell (R-Ky.), who has warned Alabama Republican Senate candidate Roy Moore that he will face an ethics investigation over sexual harassment allegations if he wins a special election next week, joined Democrats in calling for Franken to step down.

“The near daily barrage of allegations of sexual misconduct against Sen. Franken are extremely concerning to all of us in the Senate. While the Senate ethics committee is reviewing these serious allegations, it now appears that Sen. Franken has lost the support of his colleagues, and most importantly, his constituents," McConnell said. "I do not believe he can effectively serve the people of Minnesota in the U.S. Senate any longer.”